Theoretical Study On Esterification Reactions In Wine And N-asylation Reaction Catalyzed By NHC

The changes in aroma composition of wine in bottled aging wine not only invole in a variety of chemical reactions,but also have different reaction mechanisms.Although some chemical reactions and mechanisms in bottled aging wine have been explored by previos studies.howver,some of them may reveal the inherent regularity of aroma changes of the bottled aging wine is very limited.The disserations on changes of esterification-related compositions,such as carbolic acid,alcohol and ester in bottled aging wine,from computional chemistryas the storage time is prolonged,have been not found.esterification and hydrolysis reversible reaction In gas phase,water,acid medium,involved in reaction mechanisms,perference reaction pathways,determined-rate barriers etc,between carbolic acid,alcohol and ester in bottled ageing wine,were performed by the density functional method in computational chemica.All calculations were carried out using Gaussian 09 procedure.Obtained results as follows:(1)In gas phase,the geometrical structures of carboxylic acid,alcohol and ester in the esterification reversible reaction were optimized using the B3 LYP functionalin conjunction with the 6-31G(d,p)basis set.In the proposed reaction pathway R1 a,the esterification takes place via the four-membered ring transition state Ts R1 a.The oxygen in the hydroxyl group of alcohol nucleophilically attacks to the carbonyl group of acetic acid,and simultaneously,the hydrogen in the hydroxyl group of alcohol is abstracted by the oxygen in the carboxylic group of acetic acid.The activation free energy barrier for TSR1 a was calculated to be 47.4kcal/mol,indicating that it is not energy favorable.In addition to pathway R1 a,we also considered pathways R1 b,R1c,and R1 d.In all the three possible reaction pathways,the transition states have six-membered ring conformations.The corresponding activation energy barriers for TSR1 b,TSR1c and TSR1 d were calculated to be 47.3,46.0 and 43.2 kcal/mol respectively,which are so high that the reaction cannot proceed via these pathways;In water solvent,the corresponding free energy barriers for transition states R1 a,R1b,R1 c,and R1 d are 56.6,58.4,62.2,and 60.1 kcal/mol,respectively at B3LYP/6-311+g(2df,2pd)level with the appropriate IEFPCM continuum solvation model.These data demonstrate that pathways R1 a,R1b,R1 c,and R1 d are also not energetically favorable in water solvent.(2)In the acid medium,the protonated water can act as acid to catalyze the esterification reaction and the whole reaction consists of two steps: the protonated water can first transfer the proton to the hydroxyl group of acetic acid,and simultaneously,the C-O bond is breaking leading to the highly active acylium intermediate,which can subsequently react with the alcohol producing the ester.By calculations,the energy barriers for generation of acylium cations catalyzed by H3O+ are 18.0,16.7,14.3 kcal/mol for CH3 COOH,CH3(CH2)4COOH and C7H15 COOH,respectively,which demonstrate that the longer the chain of carbolic acid,the easier the reaction.In comparison,the barriers of acylium cations arising from interaction between CH3 COOH,CH3(CH2)4COOH,C7H15 COOH and C2H5OH2+ are 22.2,22.0,20.7 kcal/mol respectively,demonstrating the same tendency as above.In addition,the results also show that for the generation of the acylium ion,the protonated-water-mediated reaction pathway always has lower free energy barrier than the protonated-alcohol-mediated pathway,indicating that protonated-water-mediated pathways are more energetically favorable than protonated-alcohol-mediated pathways.Moreover,the longer the chain of the acid,the lower the reaction energy barrier.Futherly,ester hydrolysis reactionis are reverse reaction of esterification.the ester(CH3COOCH2CH2CHCH3CH3)from alcohol with branched chain have lower energy barriers(20.9 kcal/mol)for its hydrolysis.Therefore,hydrolysis is faster than esterification for the 3-methyl-butyric acid acetate leading to the content of 3-methyl-butyrate falling with the aging time prolonged as barrier of hydrolysis less than that of esterification.Otherwise,opposite trend for the content of ethyl acetate.While content of hexyl hexanate and ethyl octanate are increasing and then falling,as esterification is forming during 0,2,6 ageing years old and hydrolysis is proceeding during 6,9 years old.(3)the mechanisms of reaction for acetic acid reacts with ethanol to produce ethyl acetate and water,conjuncting with gas phase,aqueous media(AM)and acid-catalyzed environment(AE)and closing to wine ageing conditons(water 85%,v/v;ethanol 13~14%,v/v;PH=3.6),especially the rate constant and relative ratios among their rate-determinine step,are poorly understand.In this work,a systematic theoretical calculation has been performed on reaction between acetic acid and ethanol.Multiple possible reaction pathways(R1-R12)leading to two different transition states and an intermedia have been characterized.the calculated results reveal that acid-catalized environment is more likely to initiate the esterification or hydrolysis by water or ethanol cation attack on the carboxylic acid hydroxyl oxygen or ester alkyl oxygen leading to acylium ion intermedia,which is remarkably different from the results of path R1-R8 with four or six ring transition,which reasonably explains the observed experiment basing on Cabernet Gernischt bottle ageing red wine with 0,2,6,9 year old.The pathway via acylium cation intermedia is demonstrated to be at pronounced rate constant(0.9701,0.9632,0.9677,0.96459)and relative rate(1,0.9928,0.9975,0.9943)by our calculation,and the shared intermedia(acylium cation)of hydrolysis and esterifiction is the key of possibility of the reaction path(R9-R12).These further explains the data of the experimental observation of Cabernet Gernisch dry red wine for 0,2,6 and 9 years old.(4)density functional theory(DFT)calculations were performed to elucidate the detailed reaction mechanismfor N-heterocyclic carbene(NHC)-catalyzed oxidative N-acylation reaction of amides with aldehydes affording imide products.the reaction is initiated by the nucleophilic attack of NHC to aldehydes formingzwitterionic intermediate,which can then form Breslow intermediatevia proton transfer.The Breslow intermediate can then be oxidized affording the oxidative intermediate,which can then go through 1,2-addition with the deprotonated N-sulfonylcarboxamides.Subsequently,elimination of NHC catalyst produce the final imide product.results reveal that the proton in N-sulfonylcarboxamides is probably abstracted by base t-Bu OKor DPQH,and the deprotonation process is barrier-less.Moreover,for the second step,i.e.the formation of Breslow intermediate,direct proton transfer is impossible to occur.On the contrary,the results reveal thatt-Bu OH can mediate the proton transfer in this step and significantly lower the energy barrier to 24.1 kcal/mol,which is also the highest energy barrier for the whole reaction.Therefore,the reaction can happen with its lower barrier.Finally,according to our calculated results,protonated-water-mediated esterification and ester hydrolysis pathways are most energetically favorable.Moreover,our calculations also reveal that esterification can proceed more easily than ester hydrolysis,which is in agreement with the experimental observations in Cabernet Gernischt dry red wine with 0,2,6 and 9 years old.